In the prior art there are numerous processes which are directed to the solidification of ceramic gels by employing various freezing and thawing techniques. Schorger, U.S. Pat. No. 1,949,360, discloses a process for producing aluminosilicate base-exchange gels by freezing mixed gels formed from sodium aluminate and sodium silicate. Schorger used a freeze-thaw technique and preferred a freezing temperature low enough to form ice crystals but not so low as to freeze the water in the ice-compacted gel. Water expands approximately 9 volume % on freezing and thus expansion would adversely effect the strength of the gel granules after thawing and drying according to Schorger. Mahler and coworkers have worked with directional solidification of silicic acid gels. See Mahler U.S. Pat. Nos. 4,122,041 and 4,230,679, W. Mahler, M. F. Bechtold, "Freeze-formed silica fibers", Nature, Vol. 285, No. 5759, May 1, 1980, pp 27-28, and W. Mahler and V. Chowdhry, "Morphalogical Consequences of Freezing Gels", published in Ultastructure Processing of Ceramics, Glasses, and Composites, edited by L. L. Hench and D. R. Ulrich, John Wiley and Sons (1984), pp. 207-217. After freezing, the polysilicic acid undergoes accelerated, concentration dependent polymerization and is rendered insoluble. Silicic acid will polymerize at room temperature under acidic conditions. The aging time, concentration and pH have a great effect on the range of product shapes obtained on freezing. Typical gels must be aged for 6 days before fibers can be obtained from them.
An extensive review of fiber formation by directional freezing of gels is discussed in T. Maki et al., "Formation of Oxide Fibers by Unidirectional Freezing of Gel", Bull Inst. Chem. Res., Kyoto University, Vol. 64, No. 4 (1986) pp. 292-305. Three requirements for fiber formation are discussed. First, the starting sol must contain oxide particles larger than a certain size which do not come out of the cellulose tube in dialysis. The concentration of oxide component must be sufficiently high to fill the spaces between fibrous ice crystals, so that the oxide fibers in the frozen gel might be continuous. (The sol is converted to a gel by means of dialysis which is a slow process often taking days.) The oxide particles of the sol must be mobile so that they may be rearranged into the spaces between ice fibrous crystals upon unidirectional freezing of gel. Second, dialysis must be carried out for a long time sufficient for removal of the electrolytes, such as chloride, acetate or alkali ions. If a small amount of electrolytes still remain after dialysis, oxide particles might not make chemical bonds with each other in the freezing process. However, for some oxides, the dialysis time is limited by possible cracks in the gel cylinder, since the continuity of fibers may be interrupted by such cracks. The third requirement has to do with solidification conditions. The rate of immersion of gel cylinder into the cold bath should be in an appropriate range, so that cellular growth of ice fibrous crystals might take place. The preceding three requirements are general to all oxides. The reference discusses these requirements relative to gels formed from alumina sols. The alumina sols ranged from from 0.5 to 2.0 moles/liter; were stabilized with NO.sup.-, Cl.sup.- or CH.sub.3 COO.sup.- ; and had starting pH's ranged between 3.5 and 5.0. The sols were transformed to gels by dialysis in cellulose tubes with distilled water at 25.degree. C. for times up to 30 days. Unidirectional freezing was accomplished by lowering the gel cylinders into a cold bath at -78.degree. C. at rates of 3 to 13 cm/hr. Not all gels will produce fibers. A dialysis time of 18 to 30 days was required to convert the sols to firm gels suitable for freezing. Freezing ranges lower than 3 cm/hr or higher than 13 cm/hr produced granules of short fibers. Rates between 6 and 9 cm/hr produced continuous fibers for the specific sol studied. Maki et al have also prepared titania and zirconia fibers using the gel solidification and thaw technique.
Kokulo et al, "Preparation of Amorphous ZnO.sub.2 Fibers by Unidirectional Freezing of Gel," Journal of Non-Crystalline Solids, 56 (1983), 411-416 disclose a process for producing long amorphous fibers by the unidirectional freezing of zirconia hydrogel. Other gel freezing processes are disclosed by Sakka in "Formation of Glass and Amorphous Oxide Fibers from Solution" Mot. Res. Soc. Syn. Proc., Vol. 32 (1984), pp 91-99 and EP Pub. No. 371,B95.
EP 356,461 discloses forming a sintered article from particulate material by freezing a mixture of freezable liquid, a gelling material, and ceramic particles. It does not disclose any sols.
As is apparent from a review of each of the above discussed pieces of prior art, the preparation of gels suitable for directional solidification is tedious, often requiring days of dialysis time. Also, the solidification rate of gels must be exceeding slow so as to be able to form useful products. Thus a gel freeze-thaw technique would be difficult to scale up.
Accordingly, it is an object of the present invention to develop a process for producing ceramic flake products by the solidification of ceramic sols, rather than gels.
It is another object of the invention to employ a solidification process which is easy to scale-up, relatively fast, and easy to commercialize.
It is still another object of this invention to produce ceramic flakes of various sizes and shapes.